If I purchased a hotel stay via Orbitz (or other hotel booking site) which clearly said breakfast was included in the rate, I'd definitely call Orbitz first (or better yet, email so that you have a written record of the conversation) and press for my purchased rate as long as I had a screen shot confirming that I had purchased the breakfast rate for the room.

If all else fails, show the confirmation to the hotel when you check in. I agree with the previous poster that it might be worth more to the hotel to honor a free breakfast than experience that much customer ill will for a lack thereof.